Re: [PATCH] KVM: x86: optimize local IRQ disable time before vmentry
From: Paolo Bonzini
Date: Thu Mar 09 2017 - 07:34:16 EST
On 09/03/2017 13:16, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> Commit b95234c84 (kvm: x86: do not use KVM_REQ_EVENT for APICv interrupt
> injection) disables interrupts before setting vcpu->mode to fix an race:
> | The IPI for posted interrupts may be issued between setting vcpu->mode =
> | IN_GUEST_MODE and disabling interrupts. If that happens,
> | kvm_trigger_posted_interrupt returns true, but smp_kvm_posted_intr_ipi
> | doesn't do anything about it. The guest is entered with PIR.ON, but the
> | posted interrupt IPI has not been sent and the interrupt is only delivered
> | to the guest on the next vmentry (if any).
> However the race has already been fixed by the side-effects of moving the
> RVI update after IN_GUEST_MODE in the commit:
> - The IPI for posted interrupts is issued after setting vcpu->mode =
> IN_GUEST_MODE and disabling interrupts, the posted interrupt IPI is
> delayed until the guest enters non-root mode; it is then trapped by
> the processor causing the interrupt to be injected.
> - The IPI for posted interrupts is issued between setting vcpu->mode =
> IN_GUEST_MODE and disabling interrupts, the movement of the RVI update
> after IN_GUEST_MODE in the commit will catch the new PIR, and set RVI.
> This patch tries to reduce the local IRQ disable time by restoring the
> place of disable local IRQ in order to improve host kernel responsibility
> to some degree.
This should not make any difference; we're talking of a few dozen clock
cycles, and it's quite possible that the extra chance of getting a PI
interrupt in the host negates it.
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> arch/x86/kvm/x86.c | 8 ++------
> 1 file changed, 2 insertions(+), 6 deletions(-)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1faf620..9d97041 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -6857,12 +6857,6 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> - /*
> - * Disable IRQs before setting IN_GUEST_MODE. Posted interrupt
> - * IPI are then delayed after guest entry, which ensures that they
> - * result in virtual interrupt delivery.
> - */
> - local_irq_disable();
> vcpu->mode = IN_GUEST_MODE;
> srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> @@ -6881,6 +6875,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> + local_irq_disable();
> * This handles the case where a posted interrupt was
> * notified with kvm_vcpu_kick.